Vulnerability Note VU#577193

POODLE vulnerability in SSL 3.0

Overview

Many modern TLS clients can fall back to version 3.0 of the SSL protocol, which is vulnerable to a padding-oracle attack when Cypher-block chaining (CBC) mode is used. This is commonly referred to as the "POODLE" (Padding Oracle On Downgraded Legacy Encryption) attack.

Description

CWE-327: Use of a Broken or Risky Cryptographic Algorithm - CVE-2014-3566

Multiple implementations of SSL 3.0, including the implementation in OpenSSL up to version 1.0.1i, support the use of CBC mode. However, SSL 3.0 is vulnerable to a padding-oracle attack when CBC mode is used. A successful padding-oracle attack can provide an attacker with cleartext information from the encrypted communications.

Additionally, many modern TLS clients still support the ability to fall back to the SSL 3.0 protocol in order to communicate with legacy servers. A man-in-the-middle attacker may be able to force the protocol version negotiation sequence to downgrade to SSL 3.0, thereby opening up the opportunity to exploit the padding-oracle attack.

Impact

An adjacent, unauthenticated attacker may be able to derive cleartext information from communications that utilize the SSL 3.0 protocol with CBC mode.

Solution

OpenSSL has fixed the issue in OpenSSL versions 1.0.1j, 1.0.0o, and 0.9.8zc. For other implementations of the protocol, please check with the appropriate maintainer or vendor to determine if the implementation is affected by this issue. Additionally, consider the following workaround:

Use TLS_FALLBACK_SCSV

If disabling SSL 3.0 is not possible, TLS client and server implementations should make use of the TLS_FALLBACK_SCSV cipher suite value to prevent man-in-the-middle attackers from forcing unnecessary protocol downgrades.